Although the majority of pruning in the garden takes place in late winter or early spring, there is a group of trees and shrubs that can be pruned in midsummer, when they are semi-dormant. This includes yews, arborvitae, hemlocks, cedars and junipers. These evergreens have a random-branching growth pattern.
In midsummer you can lower the height of arborvitae by cutting it back (by no more than 20 percent) to a lower branch crotch. Then lightly prune the branch tips of lower limbs to balance the overall shape. Do not cut into the dead zone of older arborvitae, because it cannot create new growth.
Prune junipers to keep them in shape by removing wayward branches. Never prune into the dead zone at the plant’s center.
Shear yews and hemlocks to keep them in shape throughout the summer, but stop by autumn so that they won’t make a flush of new growth that won’t have time to harden before cold temperatures arrive.
See Horticulture‘s downloadable Smart Gardening Techniques: Summer Tasks